Deposition of Coatings on Substrates

ABSTRACT

A process and apparatus are disclosed for the deposition of a layer of a first material onto a substrate of a second material. Powder particles of the first material are entrained into a carrier gas flow to form a powder beam directed to impinge on the substrate. This defines a powder beam footprint region at the substrate. The powder beam and the substrate are moved relative to each other to move the powder beam footprint relative to the substrate, thereby to deposit the layer of the first material. A laser is operated to cause direct, local heating of at least one of a forward substrate region and a powder beam footprint region. The laser is controlled to provide a spatial temperature distribution at the powder footprint region of the substrate in which the local temperature of the substrate is in the range 0.5Ts to less than Ts in a volume from the surface of the substrate at least up to a depth of 0.2 mm from the surface of the substrate and not more than 0.25Ts at a depth of 1 mm from the surface of the substrate, wherein Ts is the solidus temperature (in K) of the second material.

BACKGROUND TO THE INVENTION

1. Field of the invention

The present invention relates to processes for the deposition ofcoatings on substrates, to apparatus for carrying out such processes andto products manufactured using such processes. The invention hasparticular, but not necessarily exclusive, application to modificationsof known cold gas dynamic spraying processes.

2. Related Art

Cold gas dynamic spraying (typically known as “cold spraying” andreferred to as such in the remainder of this specification) is a knownprocess involving the entrainment of powder particles of a firstmaterial in a fast flowing stream of gas (typically a non-oxidising gas)and allowing the particles to impinge on a substrate formed of a secondmaterial. In this specification, the flowing powder particles arereferred to as a “powder beam”. Under suitable conditions, the particlesfrom the powder beam adhere to the substrate to form a coating layer onthe substrate. The particles adhere to the substrate through plasticdeformation and bonding. It is to be noted that neither the particlesnor the substrate melt in this process (although some nanoscale localmelting may be allowed). This is advantageous for many materials, sincemacroscopic melting can deleteriously affect the materials properties ofthe coating and/or the substrate.

In US 2006/0133947, a cold spraying process is modified by the use of alaser to provide heating. The intention in US 2006/0133947 is to improvethe density of the coating. The powder particles have a diameter in therange 5-50 μm. Larger particle sizes typically cannot be accelerated tosuitable speeds. Smaller particle sizes, whilst able to be acceleratedto high speeds, tend to be swept away from the surface of the substratedue to a bow shock layer above the surface. The particles areaccelerated to speeds in the range 850-1200 m/s. The preferred carriergas in US 2006/0133947 is helium, because it allows the highest speedsto be obtained of any suitable gas. A laser is used to provide heatingto increase the density of the coating after deposition. The laser maybe moved with respect to the substrate, behind the powder beam appliedto the substrate, in order to provide in situ heat treatment to thecoating soon after the coating is deposited. There is no disclosure inUS 2006/0133947 of heating the substrate directly using the laser.

GB-A-2439934 discloses a cold spray process in which it is intended toheat the powder particles in the gas stream. This is said to be achievedby both heating the gas and by directing a laser along the powder beam.The present inventors consider that it is doubtful that the powderparticles are heated to a significant extent using the laser,considering the very short typical time of flight of the powderparticles in view of the very high speeds required for cold spraying.However, GB-A-2439934 also discloses that the laser heats the substrateat the point at which the powder particles impact the substrate, givingrise to the effect of improved bonding between the powder particles andthe substrate. There is no discussion in GB-A-2439934 of the powerdistribution of the laser beam.

US 2010/0068410 discloses a cold spray process in which a laser isdirected to coincide with the point of impact of the particle beam, toprovide local heating at that point. The stated aim of US 2010/0068410is to manage the energy of the particles, so that they arrive at thesubstrate with just enough energy to adhere to the substrate and arethen heated by the laser to fuse with the substrate. US 2010/0068410also suggests that heating of the gas stream may be advantageous, inorder to reduce the power requirements of the laser.

Christoulis et al (2010) [D. K. Christoulis, S. Guetta, E. Irissou, V.Guipont, M. H. Berger, M. Jeandin, J.-G. Legoux, C. Moreau, S. Costil,M. Boustie, Y. Ichikawa and K. Ogawa “Cold-Spraying Coupled toNano-Pulsed Nd-YaG Laser Surface Pre-treatment” Journal of Thermal SprayTechnology, Volume 19, Number 5, 1062-1073, 2010] disclose work on coldspraying of Al powder onto Al substrates. The carrier gas was nitrogenand the inlet gas temperature was 350° C. The Al substrates weresubjected to laser ablation treatment using two Q-switched Nd-YAG lasersoperating at a wavelength of 1.064 μm with an average power output of 40W each (270 mJ per pulse with an adjustable frequency up to 150 Hz) anda pulse duration of about 10 ns. The laser beam was directed to thesubstrate so that the laser beam passed over the substrate millisecondsprior to the cold spray jet of particles. Kulmala and Vuoristo (2008)[M. Kulmala and P. Vuoristo “Influence of process conditions inlaser-assisted low-pressure cold spraying” Surface and CoatingsTechnology, Volume 202, Issue 18, 15 June 2008, Pages 4503-4508]disclose cold spraying processes in which copper and nickel powders weresprayed with additions of alumina powder onto carbon steel substrates.The carrier gas was air, heated to 445° C. for copper and 650° C. fornickel. A 6 kW continuous wave laser was directed to heat the locationat which the powder beam reached the substrate surface. The processdescribed by this document is a low pressure process (gas pressure 6bar). Incorporation of ceramic particles (alumina powder) into thepowder beam assists in compaction of the deposited layer by mechanicalmeans. The use of air as the carrier gas tends to cause at least partialoxidation of particles in the powder beam, further reducing the qualityof the deposited layer.

SUMMARY OF THE INVENTION

The present inventors consider that the effect of the laser treatment inChristoulis et al (2010) is to clean the Al surface before powderdeposition, by at least partially removing the native oxide film on theAl surface. It is considered by the inventors that any heating of thesubstrate at significant depths from the surface of the substrate isnegligible.

The present inventors further consider that the process used in Kulmalaand Vuoristo (2008) would tend to lead to relatively low qualitydeposited layers, due to the low energy of the powder beam and due tothe requirement for the incorporation of ceramic particles in the powderbeam.

The present inventors have realised that further improvements of coldspraying processes may be possible. In particular, it would beadvantageous to reduce the process costs whilst ensuring high qualitydeposited layers. Although using helium gas is technically advantageousfor the gas stream, since it can achieve very high speeds, it is notpreferred, since it is very expensive and becoming more scarce. It isprohibitively expensive for typical industrial scale coatingapplications. It would be much preferred to be able to carry out asuitable cold spraying process using a different, more abundant gas,such as nitrogen. More preferably in some circumstances would be theability to carry out cold spraying using air.

These other gases cannot attain such high speeds as helium, but are muchmore attractive to industrial application of cold spraying in view ofcost.

From a technical perspective, the use of a gas other than helium as thecarrier gas means that the kinetic energy capable of being provided tothe powder particles is lower. Therefore the present inventors haveconsidered how these relatively low kinetic energy levels can be bestutilised in order to provide an industrial scale cold spray depositionprocess. The present inventors have considered whether it would beacceptable simply to pre-heat the particles in order that the particlesin the powder beam will adhere more successfully to the substrate. Thecarrier gas may also be heated if needed. Although such an arrangementis technically feasible, the power requirements for this are consideredto be unacceptably high. It is at present considered that too much heatenergy is wasted in this process to make it industrially viable.Furthermore, although pre-heating the particles can provide a technicaladvantage, this approach is limited because if the particles arepre-heated to a temperature which is too high, the particles tend tostick to the nozzle through which they are sprayed. It is effectivelyimpossible to heat the particles in-flight in order to avoid thisproblem, due to the speed of travel of the particles.

The present inventors have therefore devised the present invention,which aims to address one or more of the disadvantages outlined above.Preferably, the present invention reduces, ameliorates, avoids or evenovercomes one or more of these disadvantages.

Accordingly, in a first aspect, the present invention provides a coatingprocess for the deposition of a layer of a first material onto asubstrate of a second material, the second material optionally beingdifferent from the first material, the process including the steps:

-   -   entraining powder particles of the first material into a carrier        gas flow to form a powder beam directed to impinge on the        substrate, thereby defining a powder beam footprint region at        the substrate; and    -   causing relative movement of the powder beam and the substrate        to move the powder beam footprint relative to the substrate to        deposit the layer of the first material;        wherein, with reference to the relative movement between the        powder beam footprint and the substrate, there is defined a        forward substrate region forwards of the powder beam footprint        region,        the process further including the steps:    -   operating a heating means to cause direct, local heating of at        least one of the forward substrate region and the powder beam        footprint region; and    -   controlling the heating means and the relative movement of the        powder beam and the substrate to provide a spatial temperature        distribution at the powder footprint region of the substrate in        which the local temperature of the substrate is in the range        0.5Ts to less than Ts in a volume from the surface of the        substrate at least up to a depth of 0.2 mm from the surface of        the substrate and not more than 0.25Ts at a depth of 1 mm from        the surface of the substrate,        wherein Ts is the solidus temperature (in K) of the second        material.

In a second aspect, the present invention provides an apparatus for thedeposition of a layer of a first material onto a substrate of a secondmaterial, the second material optionally being different from the firstmaterial, the apparatus including:

-   -   a powder beam formation device capable of entraining powder        particles of a first material into a carrier gas flow to form a        powder beam directed to impinge on the substrate, thereby        defining a powder beam footprint region at the substrate; and    -   means for causing relative movement of the powder beam and the        substrate to move the powder beam footprint relative to the        substrate to deposit the layer of the first material;        wherein, with reference to the relative movement between the        powder beam footprint and the substrate, there is defined a        forward substrate region forwards of the powder beam footprint        region,        the apparatus further including:    -   a heating means operable to cause direct, local heating of at        least one of the forward substrate region and the powder beam        footprint region; and    -   control means operable to control the heating means and the        relative movement of the powder beam and the substrate to        provide a spatial temperature distribution at the powder        footprint region of the substrate in which the local temperature        of the substrate is in the range 0.5Ts to less than Ts in a        volume from the surface of the substrate at least up to a depth        of 0.2 mm from the surface of the substrate and not more than        0.25Ts at a depth of 1 mm from the surface of the substrate,        wherein Ts is the solidus temperature (in K) of the second        material.

The first and/or second aspect of the invention may have any one or, tothe extent that they are compatible, any combination of the followingoptional features.

In this way, the present invention allows the particles in the powderbeam to adhere in an improved manner to the substrate, at least in partbecause the control of the temperature distribution in the powderfootprint region of the substrate allows suitable control of the yieldstress and/or resilience of the second material, but without waste oftoo much heat deep into the substrate where the heat has no beneficialeffect on the formation of the layer.

Preferably, or alternatively to the first or second aspect of theinvention, the spatial temperature distribution at the powder footprintregion of the substrate is controlled so that the local temperature ofthe substrate is in the range 0.5Ts to less than Ts in a volume from thesurface of the substrate at least up to a first depth from the surfaceof the substrate and not more than 0.25Ts at a second, greater, depthfrom the surface of the substrate. The first depth is preferably atleast 10 times the average particle radius of the powder particles. Theparticle size distribution of the powder particles can be determinedusing laser diffraction in a known manner and the average particlediameter (and therefore radius) determined from the particle sizedistribution. A suitable instrument for measuring the particle sizedistribution is the Mastersizer 2000 from Malvern Instruments. Thesecond depth is typically 1 mm from the surface of the substrate.Alternatively, the second depth may be at least 40 times the averageparticle radius of the powder particles.

Preferably, the average particle diameter of the powder particles (basedon the total volume of the particles) is greater than 5 μm, morepreferably greater than 10 μm, still more preferably greater than 20 μm.Preferably, the average particle diameter of the powder particles is notgreater than 100 μm, more preferably not greater than 80 μm, still morepreferably not greater than 60 μm. Typically the average particlediameter may be about 50 μm. The particle size distribution is typicallya normal distribution. Preferably 90% or more by volume (more preferably95% or more, or 99% or more) of the particles in the powder beam have aparticle diameter within plus or minus 20 μm of the average particlediameter.

The particle size and the distribution is relevant because largeparticles cannot be accelerated to high enough speeds to adhere to thesubstrate. Smaller particles cannot penetrate the bow shock above thesubstrate.

The choice of a temperature profile linked to a depth of the substrateof 10 times the particle radius arises due to the inventors' insightthat the impact of an arriving particle can be considered to be similarto an indentation hardness test. In theories relating to indentationhardness, in particular for Brinell hardness (where the indentor has aspherical profile), it is considered that when the substrate has a depthof 10 or more times the indent depth, the substrate is thick enough thatthe hardness value obtained is not significantly affected by thesubstrate being too thin. This is explained in D. Tabor, Review ofPhysics in Technology 1 145 (1970) and Jonsson and Hogmark, Thin SolidFilms, 114 (1984) 257-269.

The powder particles in the powder beam may be considered to haveaverage kinetic energy Ek (Ek optionally varying with position). Ek ispreferably selected so that without direct heating of the forwardsubstrate region and/or the powder footprint region, the powderparticles would not adhere to the substrate. This arrangement ispreferred if it is required to control the adherence of the particles tothe surface by suitable control of the heating means. This is discussedin more detail below. The use of heating to promote adherence of thepowder particles also allows the deposition efficiency (DE) to beincreased. In the context of cold spraying (or laser assisted coldspraying in the preferred embodiment), DE is defined as the mass of thecoating divided by the mass of the powder sprayed to form the coating.It is therefore a measure of the efficiency with which the powderparticles in the powder beam are used in the formation of a depositedcoating.

It is typical for the powder beam to have a distribution of Ek thatvaries with position at the powder beam footprint. For example, thedistribution may be Gaussian or near-Gaussian. Then, considering a firstregion and a second region of the powder beam footprint, Ek in thesecond region may be lower than that in the first region. For example,the first region may be closer to the centre of the powder beam than thesecond region. Preferably, where there is such variation in Ek withposition across the powder beam, the heating means is controlled so thatthe temperature distribution in the powder beam footprint region is suchthat the temperature in the second region is higher than the temperaturein the first region. This allows the yield stress in the second regionto be lower than the yield stress in the first region.

More generally, where there is variation with Ek with position acrossthe powder beam, there is preferably corresponding control of theheating means so that the heating of the forward substrate region and/orthe powder beam footprint region is tailored to at least partiallycompensate for the variation in Ek with position across the powder beam.

In this way, the effect of the variation of Ek with position in thepowder beam is compensated by spatial control of the yield stress at thepowder footprint region. This allows the adherence and the uniformity ofadherence of the particles to be improved across the powder beamfootprint region.

In a similar way, and in some cases with a greater effect, there can bevariation in the material type and/or materials properties across thepowder beam footprint. This is particularly the case for the firstcoating layer deposited on the substrate, where typically the materialof the substrate may have a relatively smooth surface finish andtherefore a low absorption of laser light, whereas the material of thedeposited coating typically has a rough surface and a high absorption oflaser light. These differences across the powder beam footprint can be(at least partially) compensated for by spatial control of the heatingmeans.

With reference to the relative movement between the powder beamfootprint and the substrate, there is defined a rearwards depositedlayer region, rearwards of the powder beam footprint region. In someembodiments, the rearwards deposited layer region may also be heated.This is preferably carried out in order to densify the deposited layerand/or to relieve residual stress in the deposited layer. The heatingmay be carried out with a separate heating means. However, preferably,the heating of the rearwards deposited layer region is direct heating bythe same heating means which also heats the forward substrate regionand/or the powder footprint region, the heating means being controlledappropriately to achieve direct heating of the rearwards deposited layerregion.

The present invention includes the situation where a single layer of thefirst material is deposited on a substrate of a second material. Thesubstrate may, for example, be a ferrous substrate, e.g. a steelsubstrate. In that case, the first material typically has a differentcomposition to the second material. The first material may be anymaterial which can be coated onto the substrate using the depositiontechniques of the present invention. For example, an anti-corrosioncoating or a wear coating may be applied.

However, in some embodiments of the invention, multiple layers may beapplied. These layers are typically applied sequentially. In that case,the previously-deposited layer is acting as the substrate for the layerbeing applied. However, the principle of the invention holds in thissituation—it is still preferred that the previously-deposited layer isheated in order to control its yield stress and to ensure that the newlayer adheres properly and uniformly.

Typically, the previously-deposited layer has different properties tothe original substrate, in terms of absorption of infra red radiation inparticular, but also in terms of thermal diffusivity, specific heatcapacity and yield stress variation with temperature. Therefore, ingeneral, the heating of the previously-deposited layer should becontrolled differently to the heating of the original substrate.

Multiple layers may be applied in this way in order to achieve arelatively thick coating of substantially uniform composition. In thatcase, for layers subsequent to the first layer, the layers are beingapplied to previously-deposited layers, and a similar heating strategycan be employed for each of these subsequent layers.

However, in some embodiments, one or more of the multiple layers mayhave a different composition to one or more other layers. This can bedone in order to form a compositionally layered structure. In that case,each layer may require a different heating strategy to the other layers.

Preferably, the heating means is a laser. Suitable high power lasers arewell known and readily available. Using a laser as the heating meansreadily allows spatial and volumetric control of the temperature profileof the substrate. Where a laser is used, the process of the preferredembodiments of the invention may be referred to as “laser-assisted coldspraying”.

In a first configuration, suitable control of the heating of thesubstrate is achieved using an optical element. The optical elementprovides a laser intensity profile at the substrate which provides therequired temperature profile in the volume of the substrate below thepowder beam footprint. The optical element may operate by one or more ofreflection, refraction or diffraction. A spatially complex laserintensity profile can be formed using such an optical element. The useof such an optical element is efficient and convenient. However, it istypically limited to the situation where the required intensity profileis temporally constant (i.e. does not change with time).

In a second configuration, suitable control of the heating of thesubstrate may be achieved by scanning the laser beam over the requiredarea of the forward substrate region, the powder beam footprint regionand/or the rearwards deposited layer region. This is typically achievedby control of a movable reflective optical element. Scanning the laserbeam in this way allows precise control of the average laser intensityprofile delivered to the substrate. It also provides the advantage ofallowing temporal control of the average laser intensity profile. Thus,the laser intensity profile can be easily controlled and changed basedon changes in conditions at the substrate, e.g. for depositingsubsequent layers on previously-deposited layers.

In some embodiments, it is preferred for the heating means directly toheat the forward substrate region but not the powder beam footprintregion. Offsetting the direct heating forwardly of the powder beamallows the heating to be more carefully controlled and avoidsinterference by the incoming powder particles. This offsetting alsotypically allows higher heating intensities to be used in heating thesubstrate, thus reducing the yield stress at the substrate more quicklyand therefore making the deposition process more efficient. For manymaterials combinations, applying the same high heating intensity at thepowder beam footprint region would not be effective, because the highheating power might risk burn-off of the incident powder. In modifiedembodiments, it is preferred for the heating means directly to heat theforward substrate region using a first intensity profile and to heat atleast part of the powder beam footprint region using a second intensityprofile, wherein the average intensity of the first intensity profile isgreater than the average intensity of the second intensity profile.

Where the heating means is a laser, preferably the laser light reachesthe substrate in a direction which is non-parallel (and non-coaxial) tothe powder beam. This allows the laser light to be directed towards thesubstrate in a manner that provides a suitable temperature distributionin the substrate without significantly overlapping with the powder beam.Further details about the relative orientation of the substrate, thesubstrate movement direction, the powder beam and the laser directionare set out further below.

Preferably, the carrier gas is not heated. If the carrier gas is heated,in some embodiments it is heated to not more than 400° C. In otherembodiments, the carrier gas may be heated to higher temperatures, e.g.up to 1000° C., in order to improve deposition characteristics forcertain materials combinations.

Preferably, the solidus temperature Ts (in K) of the substrate is atleast 1175K. More preferably, the solidus temperature Ts (in K) of thesubstrate is at least 1300K.

Preferably, when the heating means is not activated, Ek of the powderbeam is selected so that the powder particles do not adhere to thesubstrate. This allows the heating means to be the controlling factor inwhether or not a deposited layer is formed. Suitable control of theheating means with a constant powder beam flow therefore allows precisepatterning of the formation of the deposited layer on the substrate. Forexample, if the maximum flow speed of the powder beam is up to 400m.s⁻¹, suitable patterning control can be achieved. In some embodiments,control of the heating means can be much more precise than control ofthe powder beam, in terms of position and/or dimensions.

The present inventors have realised that this constitutes an independentaspect of the present invention.

Accordingly, in a third aspect, the present invention provides a coatingprocess for the deposition of a layer of a first material onto asubstrate of a second material, the second material optionally beingdifferent from the first material, the process including the steps:

-   -   entraining powder particles of the first material into a carrier        gas flow to form a powder beam directed to impinge on the        substrate, thereby defining a powder beam footprint region at        the substrate; and    -   causing relative movement of the powder beam and the substrate        to move the powder beam footprint relative to the substrate to        deposit the layer of the first material;        wherein, with reference to the relative movement between the        powder beam footprint and the substrate, there is defined a        forward substrate region forwards of the powder beam footprint        region,        the process further including the steps:    -   causing the energy Ek of the powder beam to be such that when        the heating means is not activated, the powder particles do not        adhere to the substrate;    -   selectively operating a heating means to cause direct, local        heating of at least one of the forward substrate region and the        powder beam footprint region, so that the shape and location of        the deposited layer is based on operation of the heating means.

Thus, the process has additional controllability when compared to moreconventional cold spraying. Where the heating means is a laser, veryprecise patterning of the deposited layer is possible because the laserbeam can be controlled exceptionally tightly both in terms of spatiallocation and in terms of switching on and off. Furthermore, due to thecontrol of the spatial temperature profile of the substrate using thelaser, it is possible to heat only a very narrow track on the substrate,meaning that particles will adhere only at the narrow track. In thisway, it is possible to deposit layers with a width that is smaller thanthe width of the powder beam, corresponding instead to the width of thetrack of the substrate heated to a suitable temperature.

In a fourth aspect, the present invention provides an apparatus for thedeposition of a layer of a first material onto a substrate of a secondmaterial, the second material optionally being different from the firstmaterial, the apparatus including:

-   -   a powder beam formation device capable of entraining powder        particles of a first material into a carrier gas flow to form a        powder beam directed to impinge on the substrate, thereby        defining a powder beam footprint region at the substrate; and    -   means for causing relative movement of the powder beam and the        substrate to move the powder beam footprint relative to the        substrate to deposit the layer of the first material;        wherein, with reference to the relative movement between the        powder beam footprint and the substrate, there is defined a        forward substrate region forwards of the powder beam footprint        region,        the apparatus further including:    -   a heating means operable to cause direct, local heating of at        least one of the forward substrate region and the powder beam        footprint region; and    -   control means operable to control the heating means,        wherein the apparatus is operable so that the energy Ek of the        powder beam is such that when the heating means is not        activated, the powder particles do not adhere to the substrate,        selective operation of the heating means determining the shape        and location of the deposited layer.

Typically, the deposited layer is at least 0.1 mm thick. Greaterthickness is possible, for example at least 1 mm thick, or at least 2 mmthick. Coatings of up to about 4 mm thickness are possible. Thickcoatings can be built up as a series of layers. This can help to reducethe build up of stress in the coating.

Applicable to any aspect of the invention is that fact that suitablecontrol of the heating means may allow only a very narrow track of thesubstrate to be heated to the required temperature profile. This isparticularly applicable where the heating means is a laser. In this way,it is possible to form a coating whose lateral width is smaller than thediameter of the powder beam.

The inventors have also realised that the formation of a layer of thefirst material on the substrate of the second material, combined withrelative movement of the powder beam and the substrate, means thatspecial attention should be paid to the profile of the growing layer ofthe first material. This is because the profile of the growing affectsthe absorption of energy from the laser source. In some circumstances,the profile of the growing layer can shadow parts of the growing layerfrom the laser source. This can disadvantageously lead to asymmetric andreduced deposition efficiency.

Accordingly, in a fifth aspect, the present invention provides a coatingprocess for the deposition of a layer of a first material onto asubstrate of a second material, the second material optionally beingdifferent from the first material, the process including the steps:

-   -   entraining powder particles of the first material into a carrier        gas flow to form a powder beam directed to impinge on the        substrate, thereby defining a powder beam footprint region at        the substrate;    -   causing relative movement of the powder beam and the substrate        in a movement direction to move the powder beam footprint        relative to the substrate to deposit the layer of the first        material; and    -   operating a laser source to direct a laser beam along a laser        beam direction to provide a laser beam footprint to cause        direct, local heating of at least one of the forward substrate        region and the powder beam footprint region,        the laser beam direction being defined with reference to a plane        coincident with or tangential to a surface of the substrate at        the centre of the laser beam footprint in terms of an elevation        angle from the plane to the laser beam direction and in terms of        an acute azimuthal angle from the movement direction to the        laser beam direction,        wherein the elevation angle is 80° or less and the azimuthal        angle is ±60° or less.

As will be understood, it is for practical reasons not preferred for thelaser beam direction to be parallel to and particularly not coaxial withthe powder beam. Therefore typically the laser beam must be directed tothe substrate from a different direction. However, if the angle ofelevation of the laser beam (as defined above) is relatively low, thenthe growing layer of the first material at the substrate may shadowparts of that growing layer from heating by the laser. The presentinventors have found that provided the laser beam is directed at anacute azimuthal angle of 60° or less to the movement direction, thisshadowing effect can be advantageously reduced.

In some embodiments, it is preferred for the elevation angle to be 75°or less, 70° or less, 65° or less or 60° or less. Preferably theelevation angle is 30° or more.

In some embodiments, the movement direction may be variable, in order toprovide variation in the shape of the deposited layer. As will beunderstood, the movement direction may be defined by absolute movementof the substrate alone, by absolute movement of the powder beam alone,or by a combination of movement of the substrate and the powder beam. Inthese circumstances, it is preferred for the laser source to be moveablerelative to the powder beam, in order to preserve the elevation angle of80° or less and the azimuthal angle of ±60° or less, with reference tothe varying movement direction.

In a sixth aspect, there is a provided an apparatus for carrying out theprocess of the fifth aspect, wherein there are provided at least twolaser sources arrayed around the powder beam footprint, each providing adifferent laser beam direction and each laser beam direction satisfyingthe requirement of an elevation angle of 80° or less and an azimuthalangle of ±60° or less.

Preferably, where there are two or more laser sources as set out above,the azimuthal angle for at least one of the laser sources is non-zero.More preferably, the azimuthal angle for said two or more laser sourcesis non-zero, e.g. 5° or greater.

Providing at least two laser sources in this way, operating alongdifferent directions, allows the apparatus to reduce the risk ofunwanted differential heating of the substrate, thereby allowing theformation of a layer of improved uniformity.

In a seventh aspect, there is a provided an apparatus for carrying outthe process of the fifth aspect, wherein there are provided at leastthree laser sources arrayed around the powder beam footprint, theangular spacing between the laser sources being 120° or less. In oneparticularly preferred embodiment, the angular spacing between angularlyadjacent laser sources is about 120°. Where more than three lasersources are provided, the angular spacing between angularly adjacentlaser sources is less than 120°.

This array of laser sources allows the movement direction of thesubstrate relative to the powder beam to be in any arbitrary direction,since the requirement of the azimuthal angle to be ±60° or less can thenbe met for any movement direction. This allows the laser sources, ifdesired, to be fixed in position with respect to the powder beam, whichcan simplify the construction and operation of the apparatus.

The third, fourth, fifth, sixth and/or seventh aspect of the inventionmay be combined with the first and/or second aspect of the invention.Additionally, any one of (or any combination of) the optional featureset out with respect to the first and second aspect may be combined withthe third, fourth, fifth, sixth and/or seventh aspect.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of a laser-assisted cold spraydeposition process in which the laser beam (Gaussian intensity profile)and the powder beam are coaxial.

FIG. 1A shows a schematic plan view of the arrangement of FIG. 1.

FIG. 2 shows a plot of the measured temperature profile for the surfaceof the substrate for the arrangement of FIGS. 1 and 1A along the x-axisthrough the centre of the laser beam.

FIG. 3 shows a schematic side view of a laser-assisted cold spraydeposition process in which the axis of the laser beam (Gaussianintensity profile) is displaced forwardly of the axis of the powderbeam.

FIG. 4 shows a schematic plan view of the arrangement of FIG. 3.

FIG. 5 shows a plot of the measured temperature profile for the surfaceof the substrate for the arrangement of FIG. 3, measured in a similarmanner as FIG. 2.

FIG. 6 shows a schematic side view of a laser-assisted cold spraydeposition process in which the laser beam has a tailored intensityprofile.

FIG. 7 shows a schematic plan view of the arrangement of FIG. 6.

FIG. 8 shows a plot of the measured temperature profile for the surfaceof the substrate for the arrangement of FIG. 6.

FIG. 9 shows a schematic side view of a laser-assisted cold spraydeposition process in which the laser beam is scanned at the substratein order to provide a required intensity profile.

FIG. 10 shows a schematic plan view of the arrangement of FIG. 9.

FIG. 11 shows plots of measured temperature profiles for the surface ofthe substrate for the arrangement of FIG. 9.

FIG. 12 shows a schematic view of an arrangement for modellingtemperature profiles.

FIGS. 13-15 show the temperature profile results of modelling of a priorart disclosure.

FIGS. 16-18 show the temperature profile results of modelling of anembodiment of the invention.

FIG. 19 shows a cross sectional view of a cold spray deposition processaccording to an embodiment of the invention.

FIG. 20 shows a plan view of the process of FIG. 19.

FIG. 22 shows a schematic cross sectional view of the deposition of atitanium track layer on a steel substrate, the view being taken in adirection across the width of the track.

FIG. 22 shows an optical micrograph of the cross section of the trackillustrated in FIG. 21.

FIG. 23 shows a cross sectional view of a cold spray deposition processused in another embodiment of the invention.

FIG. 24 shows a schematic plan view of the arrangement of FIG. 23.

FIG. 25 shows one mode of operation of the embodiment of FIG. 23.

FIG. 26 shows a schematic plan view of the arrangement of FIG. 25.

FIG. 27 shows the particle size distribution of a Sn powder.

FIG. 28 shows a schematic plan view of a rectangular track deposited ona substrate to illustrate the effect of azimuthal angle.

FIG. 29 shows a cross sectional micrographic view of the track taken forthe track deposited in direction D1 in FIG. 28. FIG. 30 shows a crosssectional micrographic view of the track taken for the track depositedin direction D2 in FIG. 28.

FIG. 31 shows a cross sectional micrographic view of the track taken forthe track deposited in direction D3 in FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND OPTIONALADDITIONAL FEATURES OF THE INVENTION

Preferred embodiments of the invention will now be described, withreference to the drawings. Additionally, some arrangements are describedand illustrated which are outside the scope of protection, but they aredescribed and illustrated in order to provide a fuller understanding ofthe invention.

Cold spraying (CS) is a process in which high velocity particles impactand bond onto a substrate when their velocities are above a criticalvalue. Achieving this critical value typically involves the use of ahigh mach number gas (e.g. helium) and gas heating. In the preferredembodiments of the present invention, localised heating of the substratefacilitates a reduction of the critical velocity allowing for the use oflower cost carrier gas (e.g. nitrogen) and/or a reduction in the amountof gas heating required. Both of these significantly reduce thepotential cost of the process implementation.

FIGS. 1 and 2 illustrate an arrangement which is presented for referencein order to assist in an understanding of the preferred embodiments ofthe invention.

In FIG. 1, a substrate 10 is provided. In many of the embodiments of theinvention, substrate 10 is formed of steel. Steel is present here asbeing used in an embodiment of the invention but it will be understoodthat this invention is not necessarily limited to the use of steelsubstrates. However, other substrate materials are contemplated and canbe used. A layer 12 is deposited on the substrate 10 by the processdescribed in more detail below.

The substrate is moved in direction A. A powder beam 14 is formed in aknown manner by entraining powder particles of typical diameter 5-50 μminto a high speed flow of an inert carrier gas such as nitrogen andejecting the powder beam from a suitably-located nozzle (not shown). Alaser source directs high intensity laser light in the form of a laserbeam 16 to be coaxial with the powder beam. The temperature of thesurface of the substrate is measured using a pyrometer.

Typically, the kinetic energy Ek of the powder particles in the powderbeam 14 is not uniform across the diameter of the powder beam. Instead,it is typical for the powder particles towards the centre of the powderbeam to have higher energies than those towards the outside of thepowder beam. The distribution of Ek across the diameter of the powderbeam may, for example, be Gaussian or near-Gaussian.

Similarly, in typical arrangements, the distribution of intensity acrossconventional laser beams is not uniform but instead is also Gaussian ornear-Gaussian, being greater towards the centre of the beam.

FIG. 2 shows plots of measured temperature profiles for the surface ofthe substrate for the arrangement of FIG. 1. FIG. 2 is aligned with FIG.1, so that the zero on the x-axis is aligned with the centre of thelaser beam 16. The temperature distribution at the substrate along thex-axis is shown in FIG. 2. This demonstrates that the highesttemperature is found behind the powder beam. This is wasteful andinefficient. Also, there is insufficient heating forward of the powderbeam to allow the substrate to be conditioned to achieve good adherenceand compaction of the incoming particles. Furthermore, the depositedlayer typically has a different (and typically higher) absorptionproperty for the laser light. This can result in the deposited layerbeing subjected to a temperature which is too high, leading to unwantedmelting or even removal of the deposited layer.

FIG. 3 shows a schematic side view of a laser-assisted cold spraydeposition process in which the axis of the laser beam is displacedforwardly of the axis of the powder beam. This is a modification of thearrangement of FIG. 1. The laser beam once more has a Gaussian intensityprofile. FIG. 4 shows a schematic plan view of the arrangement of FIG.3. FIG. 5 shows plots of measured temperature profiles for the surfaceof the substrate for the arrangement of FIG. 3. Again, FIG. 5 is alignedwith FIG. 3, so that the zero on the x-axis is aligned with the centreof the laser beam 16. The temperature distribution at the substratealong the x-axis is shown in FIG. 5. This demonstrates that the highesttemperature is found at the centre of the powder beam. This assists inthe generation of a suitable temperature profile at the substrate toensure good adherence of the incoming powder particles onto thesubstrate without waste of the laser energy. Furthermore, displacing theaxis of the laser beam forwardly of the axis of the powder beam meansthat less laser energy is directed to the already-formed (and typicallyhigh absorbent) layer 12, meaning that there is a reduced risk ofmelting or burning away of the deposited layer 12.

FIG. 6 shows a schematic side view of a laser-assisted cold spraydeposition process which is a modification of the arrangement of FIG. 3in that the laser beam 18 has a spatial intensity profile that istailored to further improve the deposition characteristics in theprocess. As shown in FIG. 7 (plan view), laser beam 18 has a forwardportion 18 a which has a relatively high intensity and a rearwardsportion 18 b which has a relatively low intensity. Forward portion 18 ais forwards of the powder beam, meaning that the forward portion doesnot overlap with the powder beam footprint. The effect of this is thatthe heating provided by forward portion 18 a is only dependent on theabsorption of the laser light by the surface of the substrate 10.Rearwards portion 18 b of the laser beam overlaps with the powder beamfootprint and with part of the deposited layer that is rearwards of thepowder beam footprint. In view of the higher absorption at the depositedlayer, the intensity of the rearwards portion of the laser beam iscorrespondingly lower, in order to avoid melting or removal of thedeposited layer. Heating of the deposited layer to a limited extent canassist with compaction and/or stress relief of the deposited layer.

Control of the spatial intensity of the laser beam 18 is provided by anoptical element (not shown) that is typically a refractive opticalelement. However, a diffractive or reflective optical element may beused. The control provided by such an optical element is typically fixedcontrol—i.e. the resultant spatial intensity profile of the laser beamis typically fixed for a particular optical element. However, it ispossible to use different optical elements (providing different spatialintensity profiles) for different coating conditions, e.g. differentsubstrates, different coating materials, etc. Swapping suitable opticalelements in and out of the system may be done in an automated manner,e.g. robotically (for safety and reproducibility reasons).

FIG. 9 shows a schematic side view of a laser-assisted cold spraydeposition process which is a modification of the arrangement of FIG. 6in that the required laser intensity profile is provided by suitablecontrol of scanning of the laser beam 20. As shown in FIG. 10 (planview), the laser beam footprint has a forward portion 20 a which has arelatively high intensity and a rearwards portion 20 b which has arelatively low intensity. In a similar manner to the arrangement ofFIGS. 6 and 7, forward portion 20 a is forwards of the powder beam,meaning that the forward portion does not overlap with the powder beamfootprint and so can have a high intensity to ensure that a sufficientamount of laser energy is absorbed by the substrate. Rearwards portion20 b of the laser beam footprint has a lower intensity because itoverlaps with the powder beam footprint and with part of the depositedlayer that is rearwards of the powder beam footprint.

Suitable scanning optics will be well known to the skilled person inorder to provide suitable control of the spatial intensity of the laserbeam footprint. The advantage of this arrangement in comparison to thearrangement of FIGS. 6-8 is that the relative intensity of the differentparts of the laser beam footprint can be controlled in real time and canbe adjusted according to the circumstances. This allows the laserassisted cold spray deposition process to be extremely flexible in termsof the materials that can be deposited and the substrates that suchmaterials can be deposited on.

Accordingly, in these preferred embodiments of the invention, the laseris fired in the vicinity of the powder beam footprint in order toincrease the local temperature reducing the yield stress of the materialof the substrate (i.e. the original substrate and/or any previouslydeposited particles). Localised spatial control of the heat inputimproves (and in the most preferred embodiments, maximises) the processefficiency. This is particularly apparent in the deposition of the firstlayer, were a significant amount of energy is typically needed ahead ofthe powder beam to enable adhesion, whereas overheating of the trailingedge can result in coating damage due to higher absorption levels.

This technology provides a very significant improvement in theapplication of cold spraying in view of the cost reductions andincreases in coating speed that are possible. The technology hasapplication, for example, in the production of anti-corrosion and/oranti-wear coatings.

The process has additional controllability when compared to moreconventional cold spraying. For example, because the energy of the laserbeam heats the substrate to a temperature at which particle adherence ispossible, if the laser beam is turned off, the particles will not adhereto the substrate. This allows very precise patterning of the depositedlayer because the laser beam can be controlled exceptionally tightlyboth in terms of spatial location and in terms of switching on and off.Furthermore, due to the control of the spatial temperature profile ofthe substrate using the laser, it is possible to heat only a very narrowtrack on the substrate, meaning that particles will adhere only at thenarrow track. In this way, it is possible to deposit layers with a widththat is smaller than the width of the powder beam, corresponding insteadto the width of the track of the substrate heated to a suitabletemperature.

The present inventors have considered the disclosure of Kulmala andVuoristo (2008) in detail. They have assessed the disclosure of Kulmalaand Vuoristo (2008) based on computer modelling, in order to determinethe temperature profile of the substrate during processing.

Kulmala and Vuoristo (2008) discloses a low pressure cold spray process.Ceramic (alumina) powder particles are entrained in the powder beam inorder to assist compaction. The substrates used are low carbon mildsteel substrates.

The relevant parameters in Kulmala and Vuoristo (2008) are as follows:

-   -   Temperatures 650 to 1000° C.    -   At 650° C. laser powers 2.0 kW to 1.8 kW.    -   At 1000° C. laser powers 2.9 to 2.7 kW for multiple layers.    -   6 kW laser power available.    -   Processing speed 40 mm.s⁻¹.    -   Laser spot 5.8×23.5 mm.    -   Uniform intensity across laser spot.    -   Powder nozzle diameter 5 mm.    -   Powder beam diameter 5.8 mm.    -   Power absorption approximately 35%.

The surface temperature of a substrate can be measured using apyrometer. When the substrate is heated using a laser, a suitablefrequency filter can be used at the pyrometer to filter out thewavelength of the laser light. Power absorption can be determined byfirst measuring the reflectivity of the substrate, i.e. by measuring theintensity of light reflected from the substrate compared with theintensity of light incident at the substrate. The power absorption isthen (1-reflectivity)×100, to express as a percentage. The temperaturedistribution in the substrate can then be determined in a known mannerbased on the thermal materials properties of the substrate, describedbelow.

In FIGS. 13-18, temperature profiles are shown. The contours are 100Kapart. In each of the figures, the lowest temperature contour is 373K.In FIGS. 13 and 15 the highest temperature contour is 973K. In FIG. 14the highest temperature contour is 773K. In FIGS. 16-18 the highesttemperature contour is 1273K.

It should be noted that the modelling work reported here is materialdependent. Here, the substrate is steel. The material to be deposited inthe modelling is Ti.

FIG. 12 shows a schematic view of an arrangement for modellingtemperature profiles. A stationary laser source (not shown) provides alaser beam 50 that is passed through suitable beam optics 52 to form alaser footprint 54 of a specific shape on a substrate 56. The substrate56 is moved in direction A order to provide relative movement betweenthe substrate 56 and the laser footprint 54. Axes X, Y and Z are shownin FIG. 12. The relative movement between the substrate and the laserfootprint is along axis X. The thickness direction of the substrate isalong axis Z. The centre of the laser footprint is at X=Y=Z=0. At thebeam footprint, the laser power is delivered to the substrate. The beamfootprint width is Sw. The beam footprint length is SI. The substratethickness is Mt.

Based on the disclosure of Kulmala and Vuoristo (2008), the laserintensity is uniform across the laser footprint.

The low carbon mild steel substrate used in Kulmala and Vuoristo (2008)is assumed to have the following properties:

-   -   Thermal conductivity 25.6 W.m⁻¹.K⁻¹    -   Specific heat 925 J.kg⁻¹.K⁻¹    -   Density 7640 kg.m⁻³    -   Melting temperature 1765K    -   Boiling temperature 3000K

Ambient temperature is assumed to be 20° C.

The resultant isotherms are shown in FIGS. 13, 14 and 15. FIG. 13 showsthe temperature profile of the surface of the substrate in plan view.FIG. 14 shows the temperature profile of the substrate in a crosssection view of the y-z plane (at x=0). FIG. 15 shows the temperatureprofile of the substrate in a cross section view of the x-z plane (aty=0). Based on the disclosure of Kulmala and Vuoristo (2008), the centreof the powder beam would be at about x=+9 mm in FIG. 4.

However, the present inventors consider that the arrangement in Kulmalaand Vuoristo (2008) is inefficient. Much of the heat delivered to thesubstrate is wasted. Heating of the substrate is useful if it affectsthe adherence of the incoming powder particles. However, in Kulmala andVuoristo (2008), much of the energy is used to heat deeper regions ofthe substrate (e.g. deeper than about 0.5 mm).

The temperature at the surface of the substrate at the powder beamfootprint in Kulmala and Vuoristo (2008) is about 700° C. (973K). Thetemperature at about 0.5 mm from the surface below the powder beamfootprint is about 600° C. (873K). The temperature at about 1 mm fromthe surface below the powder beam footprint is about 400° C. (673K). Thetemperature at about 2 mm from the surface below the powder beamfootprint is about 200° C. (473K).

In an embodiment of the present invention, a titanium substrate is used.The dimension of the substrate are identical to those in Kulmala andVuoristo (2008). The relevant parameters of this embodiment are asfollows:

-   -   Maximum temperature 900° C.    -   4 kW laser power available.    -   Processing speed 500 mm.s⁻¹.    -   Laser spot diameter 6 mm.    -   Gaussian intensity profile across laser spot.    -   Power absorption approximately 40%.

The titanium substrate used in this embodiment has the followingproperties:

-   -   Thermal conductivity 6.8 W.m⁻¹.K⁻¹    -   Specific heat 564 J.kg⁻¹.K⁻¹    -   Density 4428 kg.m⁻¹    -   Melting temperature 1941K    -   Boiling temperature 3560K

The resultant isotherms are shown in FIGS. 16, 17 and 18. FIG. 16 showsthe temperature profile of the surface of the substrate in plan view.FIG. 17 shows the temperature profile of the substrate in a crosssection view of the y-z plane (at x=0). FIG. 18 shows the temperatureprofile of the substrate in a cross section view of the x-z plane (aty=0). The preferred location for the centre of a powder beam would be atabout x =+3 mm in FIG. 18.

As can be seen from these results, this embodiment of the inventionprovides suitably deep heating of the substrate in order to promote goodadhesion and density of the deposited layer but avoids the wastage ofheat into the deeper parts of the substrate below the powder beamfootprint.

With reference to FIGS. 19 and 20, there is shown a substrate 100 movingin movement direction A. Onto the substrate is directed a powder beam102 along a powder beam direction 104 and a laser beam 106 along a laserbeam direction 108. In this embodiment, the laser beam heats thesubstrate at the powder beam footprint in order to deposit a layer 110formed of the particulate material in the powder beam. The angle esubtended in FIG. 19 between the plane of the substrate 100 and thelaser beam direction is the elevation angle. As will be clear, if theelevation angle was 90°, the laser beam direction would be parallel tothe powder beam direction 102. However, in this embodiment, theelevation angle e is about 45°. Where the substrate is non-planar, theelevation angle e is defined with reference to a plane tangential tosurface of the substrate at the centre of the laser beam footprint.

As shown in FIG. 20, angle a is the azimuthal angle of the laser beamdirection 108. This is the acute angle subtended between the movementdirection A and a projection of the laser beam direction onto thesurface of the substrate 100.

Azimuthal angle a is ±60° or less from the movement direction. As willbe understood, this places a restriction on the direction in which thesubstrate can be moved relative to the laser beam direction. If it iswanted to form a deposited layer in any direction on the surface of thesubstrate, it will be necessary either to provide a laser source that ismoveable to provide the required laser beam direction, or it will benecessary to provide more than one laser beam source, to be switched inand out of operation depending on the shape of the track and hence thedirection of movement of the substrate. Given the limitation of ±60° forthe range of suitable laser beam directions for providing adequate shapefor the deposited layer, it will be understood that preferably an arrayof laser sources is provided, preferably at least 3 laser sources,angularly arranged around the powder beam footprint with an angularspacing of 120° or less.

The term “laser source” here is intended to mean a device whichfunctions to provide a laser beam along the required direction.Therefore where more than one laser source is provided, and thus morethan one laser beam is provided, it is possible for the laser beams tobe derived from the same laser. This can be done by a suitable arrangedof optical elements such as fibre optics.

As discussed above, when the laser is applied in a non coaxial mannerthe direction which the laser enters the powder beam is significant. Ithas been found that if the azimuthal angle is greater than 60° then oneor both of the deposition efficiency and track shape becomes impaired.When the laser is shaded by deposited powder the deposition efficiencyof the process is altered. This manifests as a distortion of the trackwhen the laser is primarily coming into the side of the deposit. Whenincident from the rear of the track there is a uniform drop indeposition efficiency.

FIG. 21 illustrates the situation. Here there is shown a schematic crosssectional view of the deposition of a titanium track layer 150 on asteel substrate (not shown), the view being taken in a direction acrossthe width of the track. The mass distribution 152 across the powder jetP is shown as a symmetrical, near-Gaussian distribution. However, if thelaser beam L arrives at a non-zero azimuthal angle (here 30°), then theshape of the track 150 becomes asymmetrical (as seen by X1 and X2 beingunequal), due to the differential heating at the powder footprint. InFIG. 21, the track height is Th, the powder beam footprint width is 8mm, the laser footprint is 4.5 mm and the laser power distribution is154.

FIG. 22 shows an optical micrograph of the cross section of the trackdescribed above.

FIGS. 23-26 illustrate another embodiment of the invention, adapted tocontrol the track shape and to allow the track direction to be varied atwill. Similar features already described with reference to FIGS. 19 and20 are given the same reference numbers and are not necessarilydescribed again.

In FIGS. 23 and 24, 6 incoming laser beams 206A-206F are provided alongrespective directions 208A-208F. Azimuthal angle A is about 30°, andazimuthal angle B is about −30°. As will be understood, the measurementof these azimuthal angles depends on the direction of movement of thesubstrate as shown in FIGS. 19 and 20. Azimuthal angles for beams 206C,D, E and F are not shown but can be measured according to theexplanation above. As shown in FIG. 23, elevation angle e of the laserbeams 206A and 206E is the same and is about 45°.

As shown in FIGS. 25 and 26, when the substrate is moved in direction A,trailing and side-facing laser beams 206C, D, E and F can be switchedout of operation. As will be understood, the substrate direction can bechanged and one or more of the laser beams 206C, D, E and F can beswitched into operation in order to provide a suitable heating profilefor the substrate, the powder footprint region and/or the depositedlayer. This arrangement therefore allows the substrate to move in anydirection desirable to form a deposition track, by suitable switching inand out of operation the various laser sources as needed.

FIG. 27 shows the particle size distribution of a Sn powder, showing atypical particle size distribution measured using a Malvern Mastersizer2000 instrument. The shape of the distribution is typical of powderssuitable for use with embodiments of the invention, but it is noted herethat the average particle size shown in FIG. 27 is slightly too low foroptimum suitability with the preferred embodiments of the invention.Alternative powders that can be used are, for example, Ti powder orstellite powder. Stellite is a Co—Cr alloy. Typically, the averageparticle size required for use in the invention depends to some extenton the density of the material to be sprayed. A more dense material(e.g. stellite) typically requires a finer particle size becauseparticles that are too large will not accelerate well enough in the gasjet. A suitable average particle size for stellite is about 40 μm. Aless dense material (e.g. Ti) has a coarser particle size for use in theembodiments of the invention. A suitable average particle size for Ti isabout 55 μm.

Experimental work to show the advantage of the preferred embodiment ofthe invention has been carried out, as illustrated in FIG. 28 whichshows a schematic plan view of a rectangular track 302 deposited on asubstrate 300 to illustrate the effect of azimuthal angle.

A powder beam was formed to impinge perpendicularly on a substrate asdescribed above. The laser beam had an elevation angle of about 60°. Thedirection of movement of the substrate with respect to the powder beamand laser beam was varied so as to vary only the azimuthal angle. Thespecific variation used in this experiment was to form a deposited layeralong the outline of a rectangular track, starting at position S. Thetrack was deposited first along direction D1, along one side of therectangle, with the azimuthal angle at 0°. Then the direction ofmovement of the substrate was changed and the track was then depositedalong the next side of the rectangle along direction D2, with theazimuthal angle at 90°. Next, the direction of movement of the substratewas changed again and the track was deposited along the next side of therectangle along direction D3, with the azimuthal angle at 180°. Finally,the direction of movement of the substrate was changed again and thetrack was deposited along the next side of the rectangle along directionD4, with the azimuthal angle at 270°, to return to position S.

The relative deposition efficiency for each side of the rectangle wasdetermined, on the fair assumption that the particle beam was constantand the laser power was constant during the full spraying treatment. Theresults showed that the deposition efficiency for azimuthal angle being0° was significantly better than the deposition efficiency for the otherazimuthal angles tested in the experiment.

FIG. 29 shows a cross sectional micrographic view of the track taken forthe track deposited in direction D1 in FIG. 28. FIG. 30 shows a crosssectional micrographic view of the track taken for the track depositedin direction D2 in FIG. 28. FIG. 31 shows a cross sectional micrographicview of the track taken for the track deposited in direction D3 in FIG.28. Each micrographic image is of an etched microstructure. As can beseen, track D1 is relatively symmetrical and uniform in microstructure.However, where the azimuthal angle is 90°, 180° or 270° (−90°), thedeposited tracks are asymmetrical and/or molten and partially oxidised.

The present inventors have also demonstrated the control of thedeposition of Ti powder particles when the powder beam is maintained onbut the laser is turned on and off to control the position of theformation of the deposited layer along the track. This has beendemonstrated for a constant traverse speed along the substrate of 500mm/min, the laser being turned on for 2 seconds at a time to depositindividual islands of the coating layer.

The invention is considered at present to have considerable merit forthe deposition of coatings on tubes. Typically the coating is depositedon the external surface of the tube (e.g. as an anti-corrosion coating).In this application, typically the tube is rotated and the relativeaxial position between the powder beam footprint and the tube iscontrolled to provide a continuous coating.

The invention is also considered at present to have considerable meritfor the deposition of coatings on relatively small localised area(s) ofa substrate. This is of interest in particular for the repair of surfacedefects on high value components such as turbine blades. In thisapplication, control over the azimuthal angle as defined above isconsidered to be particularly important, to allow the coating to beapplied in a desired pattern to cover the localised area as required.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

1. A coating process for the deposition of a layer of a first materialonto a substrate of a second material, the second material optionallybeing different from the first material, the process including thesteps: entraining powder particles of the first material into a carriergas flow to form a powder beam directed to impinge on the substrate,thereby defining a powder beam footprint region at the substrate; andcausing relative movement of the powder beam and the substrate to movethe powder beam footprint relative to the substrate to deposit the layerof the first material; wherein, with reference to the relative movementbetween the powder beam footprint and the substrate, there is defined aforward substrate region forwards of the powder beam footprint region,the process further including the steps: operating a heating means tocause direct, local heating of at least one of the forward substrateregion and the powder beam footprint region; and controlling the heatingmeans and the relative movement of the powder beam and the substrate toprovide a spatial temperature distribution at the powder footprintregion of the substrate in which the local temperature of the substrateis in the range 0.5Ts to less than Ts in a volume from the surface ofthe substrate at least up to a depth of 0.2 mm from the surface of thesubstrate and not more than 0.25Ts at a depth of 1 mm from the surfaceof the substrate, wherein Ts is the solidus temperature (in K) of thesecond material.
 2. The coating process according to claim 1, wherein,with reference to the relative movement between the powder beamfootprint and the substrate, there is defined a rearwards depositedlayer region, rearwards of the powder beam footprint region, therearwards deposited layer region also being heated.
 3. The coatingprocess according to claim 1, wherein a single layer of the firstmaterial is deposited on a substrate of a second material, the firstmaterial having a different composition to the second material.
 4. Thecoating process according to claim 1, wherein multiple layers areapplied sequentially, each previously-deposited layer acting as thesubstrate for the layer being applied.
 5. The coating process accordingto claim 1, wherein the first material is an anti-corrosion coating or awear coating.
 6. The coating process according to claim 1, wherein theheating means is a laser.
 7. The coating process according to claim 6,wherein control of the heating of the substrate is achieved using anoptical element, the optical element providing a laser intensity profileat the substrate which provides the required temperature profile in thevolume of the substrate below the powder beam footprint.
 8. The coatingprocess according to claim 6, wherein control of the heating of thesubstrate is achieved by scanning the laser beam over the required areaof the forward substrate region, the powder beam footprint region and/orthe rearwards deposited layer region.
 9. The coating process accordingto claim 1, wherein the heating means is operated directly to heat theforward substrate region but not the powder beam footprint region. 10.The coating process according to claim 1, wherein the heating means isoperated directly to heat the forward substrate region using a firstintensity profile and to heat at least part of the powder beam footprintregion using a second intensity profile, wherein the average intensityof the first intensity profile is greater than the average intensity ofthe second intensity profile.
 11. The coating process according to claim1, wherein the carrier gas is selected from the group consisting of:nitrogen and air.
 12. The coating process according to claim 1, whereinthe carrier gas is not heated.
 13. The coating process according toclaim 1, wherein the powder particles in the powder beam have averagekinetic energy Ek, Ek optionally varying with position across the powderbeam, and Ek is selected so that without direct heating of the forwardsubstrate region and/or the powder footprint region, the powderparticles do not adhere to the substrate, the process including the stepof selectively deactivating the heating means in order to prevent thepowder particles from adhering to the substrate.
 14. The coating processaccording to claim 1, wherein the deposited layer is at least 0.1 mmthick.
 15. The coating process according to claim 1, wherein the spatialtemperature distribution at the powder footprint region of the substrateis controlled so that the local temperature of the substrate is in therange 0.5Ts to less than Ts in a volume from the surface of thesubstrate at least up to a first depth from the surface of thesubstrate, the first depth being at least 10 times the average particleradius of the powder particles.
 16. An apparatus for the deposition of alayer of a first material onto a substrate of a second material, thesecond material optionally being different from the first material, theapparatus including: a powder beam formation device capable ofentraining powder particles of a first material into a carrier gas flowto form a powder beam directed to impinge on the substrate, therebydefining a powder beam footprint region at the substrate; and means forcausing relative movement of the powder beam and the substrate to movethe powder beam footprint relative to the substrate to deposit the layerof the first material; wherein, with reference to the relative movementbetween the powder beam footprint and the substrate, there is defined aforward substrate region forwards of the powder beam footprint region,the apparatus further including: a heating means operable to causedirect, local heating of at least one of the forward substrate regionand the powder beam footprint region; and control means operable tocontrol the heating means and the relative movement of the powder beamand the substrate to provide a spatial temperature distribution at thepowder footprint region of the substrate in which the local temperatureof the substrate is in the range 0.5Ts to less than Ts in a volume fromthe surface of the substrate at least up to a depth of 0.2 mm from thesurface of the substrate and not more than 0.25Ts at a depth of 1 mmfrom the surface of the substrate, wherein Ts is the solidus temperature(in K) of the second material.